Pressure die castings are conventionally made by injecting molten metal under high pressure into permanent dies containing cavities of the desired shape. Such process has been widely used for many years because of its fast cycling speed and accurate reproduction of cavity surface details.
The pressure die casting process characteristically employs rapid metal injection and fast solidification, and uses high compacting forces of thousands of pounds per square inch on the molten metal to reduce the size of voids which may be present due to gases trapped by the turbulent metal flow and shrinkage voids due to rapid solidification.
More recently, a process using only a few pounds of pressure on the metal has come into use and is called "low pressure die casting" to distinguish it from the higher pressure faster cycling "pressure die casting" process. Low pressure die casting is a process in which the molten metal is forced upwardly from an enclosed metal bath through a tube or stalk into a top mounted mold or die held closed by clamping means mounted above the metal bath.
Such low pressure die casting process is designed to move the molten metal slowly in unturbulent flow, and to continue the application of low pressure during slow solidification in order to achieve maximum casting density. This process cycle is measured in minutes instead of the seconds characteristic of conventional pressure die casting. However, low pressure die casting usually requires less massive, and therefore less costly clamping means for the dies and is used to make heavy walled castings requiring high density and uniform metallurgical structure, but not necessarily precise reproduction of cavity surfaces.
Pressure die castings other than low pressure die castings are made generally in two different types of machines. One is called a "hot chamber" machine and has the iron or steel metal injection chamber immersed vertically in the molten casting metal bath. It cannot normally be used for aluminum casting alloys into which the iron or steel of the pressure chamber is readily soluble, thus destroying the required compression fit of the metal injection piston and contaminating the casting alloy. A hot chamber machine also cannot be used for higher melting temperature casting alloys such as brass as the higher alloy melting temperature would lower the strength of the immersed components of the metal injection system which must be operated at high pressure. Aluminum and other high melting point alloys are therefore die cast in a so-called "cold chamber" die casting machine where the injection chamber is not immersed in the molten metal bath. In a cold chamber machine the molten casting alloy is ladled or otherwise charged for each casting cycle into the horizontally disposed pressure chamber through a pouring hole in its upper surface.
The hot chamber die casting process cycles more quickly than the cold chamber process because the molten casting alloy is replaced in the pressure chamber from the metal bath through a side hole which is uncovered after each casting cycle by the upward return stroke of the injection piston. This then provides a fresh charge of metal which only awaits the closing of the die halves before it can be injected into the die. In the present horizontal cold chamber machines, the die end of the metal injection chamber is open until the die halves are closed, making it necessary to delay the pouring of metal into the pressure chamber until after the die halves are closed. This, of course, delays the metal injection during each cycle.
In both the cold chamber and hot chamber machines, the metal injection stroke is rapid, and the mass of the injection piston, its connected actuating mechanism in the hydraulically actuated driving cylinder, and the hydraulic fluid moving into the cylinder represent a substantial mass which is suddenly arrested at the end of the die filling stroke. The result is a sudden rise in pressure in the molten metal in the die which is usually far in excess of the pressure which is needed to fill the cavity. Such sudden rise in pressure is known as the impact pressure peak.
As a consequence, the clamping mechanism must be designed sufficiently strong to contain this impact pressure peak, or the die halves will be separated slightly permitting metal to escape from the cavity at the die parting line. This flashing causes dimensional discrepancies in the castings and makes flash removal a necessity. If the clamping force is inadequate and the impact pressure peak great enough, the molten metal may be forced out with such speed that it will not solidify on the parting line faces of the die and it will escape at high velocity. All casting machines sold are therefore required to have enclosures which cover the die parting plane during metal injection.
In addition to the undesirable impact pressure peak, the injection systems are actually designed to apply a compacting pressure greater than the pressure needed just to fill the cavities. This compacting pressure is applied to the metal after cavity fill in order to minimize the size of entrapped gas inclusions in the casting and to feed solidification shrinkage of the metal in the casting cavities. This compacting pressure is applied by force exerted in the injection chamber, and it must be transmitted through the solidifying metal both there and in the gate runners, and through the gate orifice before that connection solidifies completely.
Both impact and compacting pressures are in excess of what is actually required to fill the casting cavity and both result in high cost energy consumed, excessive metal in the gate runners and gate orifice, and also in excessive cost of the casting machine injection and clamping mechanism. In the cold chamber process, the metal solidifying on the wall of the injection chamber must be collapsed by the injection piston to advance to its compacting stroke, which increases the energy required for the compacting action.